Weather data may predict cholera outbreaks

[MONTEVIDEO] Public health officials may be able to forecast cholera outbreaks months in advance by looking at temperature and rainfall data, according to a study.

Researchers looked at cholera outbreaks in Zanzibar, Tanzania, and found that they correlated with increases in temperature and rainfall.

They calculated thata one degree Celsius increase — from 23 to 24 degrees Celsius — was followed by a doubling of cholera cases four months later. And a 20 centimetre increase of rainfall resulted in a 1.6 fold increase in cholera cases two months later.

"Our model, based on these two parameters, forecasted cases of cholera in Zanzibar well, and could act as a predictor for cholera outbreaks," Mohammad Ali, a researcher at the International Vaccine Institute in Seoul, South Korea, and one of the authors of the study, told SciDev.Net.

Monthly cholera disease surveillance reports between 1997 and 2006 allowed the researchers to map epidemics over time. These epidemics were then compared with monthly environmental data for the same period, according to the study, published in June issue of TheAmerican Journal of Tropical Medicine and Hygiene.

Cholera outbreaks were closely associated with a rise in the monthly mean of minimum daily temperatures and rainfall levels, and this data could work as a reliable forecasting system for the disease, they concluded.

This would enable public health officials to prepare efficient and cost-effective interventions, such as vaccination programmes, to stem the outbreaks before they happen.

"We believe that these two climate variables — temperature and rainfall — can be applied as predictors in other regions," said Ali. "Now, we are working with the same type of data from Bangladesh. Our initial findings suggest that the amplification of cholera in this part of the world is also temperature driven."

But reliable data on cholera incidence is lacking in some developing countries, Ali added, making it impossible to develop similar forecasting models.

"I do not foresee that such a model could be developed in all countries," Ali said. "We can say, though, that neighbouring countries of Tanzania that have similar water and sanitation infrastructures may benefit from the model."

The study also says that with global warming, there is possibility of increased cholera incidence in many resource-poor regions of the world.

But Shafiqul Islam, a professor of engineering at Tufts University in the United States, who specialises in water issues, said that "the study period [six years] is too short to invoke 'climate' arguments".

"It is not clear what physical mechanisms are important for cholera outbreaks in the Zanzibar region. Without an understanding of underlying environmental processes, it will be very difficult to validate any predictive model for cholera," he said.

Peter Hotez, a tropical disease expert at George Washington University in the United States and president of the American Society of Tropical Medicine and Hygiene, said: "This is an exciting study potentially linking climate change in Africa to the emergence of cholera. Over the last few years we have seen protracted cholera outbreaks in Angola, Zimbabwe and elsewhere on the continent and this study may partly account for this increase.

"This is also a wake-up call for the possibility of other outbreaks in Africa and we need to consider expanding existing public health interventions, including cholera vaccine."